Stroke occurs in 5-10% of children with SCA, and is a devastating clinical event that results in severe motor and neurocognitive deficits. To help prevent an initial (primary) stroke, young patients with SCA can receive periodic TCD screening to identify children with elevated arterial velocities in the cerebral vasculature, which portends increased primary stroke risk. For children with time-averaged maximum velocities (TAMV) in the middle cerebral artery (MCA) or internal carotid artery (ICA) elevated to the "abnormal" range (=200cm/sec), chronic erythrocyte transfusions significantly lower the risk of primary stroke. In this setting, transfusions can prevent first stroke but have serious side-effects limiting their long-term utility. Transfusions transmit infectious agents, lead to erythrocyte alloantibody or autoantibody formation, and result in iron overload. Transfusion acquired iron overload is recognized as a source of morbidity and mortality for young patients with SCA receiving transfusions for prevention of primary stroke. Chelation therapy can help prevent iron accumulation, but is difficult to tolerate and non-compliance is common. An alternative to transfusion prophylaxis for primary stroke prevention is clearly needed, especially one that also provides an opportunity to address the issue of transfusion acquired iron overload. We propose a Phase III randomized clinical trial for children with sickle cell anemia (SCA) and abnormal Transcranial Doppler (TCD) velocities, termed the "TCD With Transfusions Changing to Hydroxyurea" (TWiTCH) trial. Our hypothesis is that hydroxyurea can maintain a similar TCD velocity as erythrocyte transfusions, and therefore serve as non-inferior therapy, for primary stroke prevention in high risk children with SCA. The primary aim of the TWiTCH trial is to compare standard therapy (transfusions) to alternative therapy (hydroxyurea) for maintenance of TCD velocities in children with SCA on chronic transfusions for abnormal TCD velocities. Additional aims of TWiTCH include comparison of standard to alternative therapy for incidence of primary stroke, determination of the frequency of non-stroke neurological events and other sickle cell-related events, management of iron overload, assessment of growth and development, recording of adverse events, and measurement of quality of life. (End of Abstract)